The present invention is directed to ink jet recording apparatuses and processes. More specifically, the present invention is directed to printhead suitable for ink jet printing processes, said printheads being coated with a water repellent material. One embodiment of the present invention is directed to an ink jet printhead comprising a plurality of channels, wherein the channels are capable of being filled with ink from an ink supply and wherein the channels terminate in nozzles on one surface of the printhead, the surface being coated with a polyimide-siloxane block copolymer. In a specific embodiment, the polyimide-siloxane block copolymer is of the formula ##STR1## wherein x and y represent the numbers of repeating segments, the dotted circles Ar.sub.1, Ar.sub.2, Ar.sub.3, and Ar.sub.4 are independently selected from aromatic groups, l and n are numbers of from 1 to about 15, m is a number of from 1 to about 100, R.sub.1 and R.sub.2 are independently selected from alkyl groups, and R.sub.3 represents an aliphatic or an aromatic group, and the polymer contains from about 1 to about 30 percent by weight of siloxane.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are different types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The surface of the printhead encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet equipment and processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, U.S. Pat. No. 4,532,530, and U.S. Pat. No. 4,774,530, the disclosures of each of which are totally incorporated herein by reference.
The present invention is suitable for ink jet printing processes, including drop-on-demand systems such as thermal ink jet printing, piezoelectric drop-on-demand printing, and the like.
In ink jet printing, a printhead is usually provided having one or more ink-filled channels communicating with an ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. These printheads form images on a recording medium such as paper by expelling droplets of ink from the nozzles onto the recording medium. The ink forms a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus. An important property of a high quality printhead array is good jet directionality. Good jet directionality ensures that ink droplets can be placed precisely where desired on the print document. Poor jet directional accuracy leads to the generation of deformed characters and visually objectionable banding in half tone pictorial images.
A major source of ink jet misdirection is associated with improper wetting of the surface of the printhead which contains the array of nozzles. One factor which adversely affects jet directional accuracy is the interaction of ink accumulating on the surface of the printhead array with the ejected droplets. Ink may accumulate on the printhead surface either from overflow during the refill surge of ink or from the spatter of small satellite droplets during the process of expelling droplets from the printhead. When the accumulating ink on the printhead surface makes contact with ink in a channel (and in particular with the meniscus of ink protruding from a nozzle), it distorts the ink meniscus, resulting in an imbalance of the forces acting on the egressing droplet, which in turn leads to jet misdirection. This wetting phenomenon becomes more troublesome after extensive use as the array face oxidizes or becomes covered with a dried ink film, leading to a gradual deterioration of the image quality that the printhead is capable of generating. To retain good ink jet directionality, wetting of the surface of the printhead desirably is suppressed. Alternatively, if wetting can be controlled in a predictable, uniform manner, jet misdirection would not be a problem. Uniform wetting, however, is difficult to achieve and maintain.
In thermal ink jet printing, a thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. The rapidly expanding vapor bubble pushes the column of ink filling the channel towards the nozzle. At the end of the current pulse the heater rapidly cools and the vapor bubble begins to collapse. However, because of inertia, most of the column of ink that received an impulse from the exploding bubble continues its forward motion and is ejected from the nozzle as an ink drop. As the bubble begins to collapse, the ink still int he channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper. The collection of ink on the nozzle containing face of thermal ink jet printheads causes all of the problems discussed above.
Ink jet printheads include an array of nozzles and may, for example, be formed of silicon wafers using orientation dependent etching (ODE) techniques. The use of silicon wafers is advantageous because ODE techniques can form structures, such as nozzles, on silicon wafers in a highly precise manner. Moreover, these structures can be fabricated efficiently at low cost. The resulting nozzles are generally triangular in cross-section. Thermal ink jet printheads made by using the above-mentioned ODE techniques typically comprise a channel plate which contains a plurality of nozzle-defining channels located on a lower surface thereof bonded to a heater plate having a plurality of resistive heater elements formed on a upper surface thereof and arranged so that a heater element is located in each channel. The upper surface of the heater plate typically includes an insulative layer which is patterned to form recesses exposing the individual heating elements. This insulative layer is referred to as a "pit layer" and is sandwiched between the channel plate and heater plate so that the nozzle-containing front face has three layers: the channel plate, the pit layer and the heater plate. For examples of printheads employing this construction, see U.S. Pat. No. 4,774,530 and U.S. Pat. No. 4,829,324, the disclosures of which are totally incorporated herein by reference.
The heater and channel plates are typically formed from silicon. The pit layer sandwiched between the heater and channel plates, however, is formed from a polymer, an example being polyimide. Since the front face of the printhead where the channels terminate in nozzles is made from different materials, a coating material, such as a water-repellent material, generally is not likely to adhere equally well to these different materials, resulting in a coating which is not uniformly ink-repellent. Thus, it is difficult to provide a surface coating which is uniformly ink-repellent in ink jet printheads formed from multiple layers.
Additionally, ink jet printers typically employ inks which contains a glycol and water. Glycols and other similar materials are referred to as humectants, which are substances which promote the retention of moisture. For a coating material to be effective for any length of time, it must both repel and be resistant to glycol-containing inks.
Further, it is difficult to apply a coating to the surface of an ink jet nozzle. While it is desirable to suppress the wetting property of the nozzle jet surface, it is undesirable to allow any coating material to enter the channels to the nozzle. A key requirement for good directionality is that the interior channel walls not be coated. If the walls of the channels become coated with ink-repellent material, proper refill of the channel is inhibited. Refill of each channel depends on surface tension and must be completed in time for subsequent volleys of drops to be fired. If the refill process is not complete by the time the next drop is fired, the meniscus may not be flush with the outer edge of the nozzle orifice, resulting in misdirection. Further, an incompletely filled channel causes drop size variability, which also leads to print quality degradation.
Thus, misdirection of droplets of ink ejected from a thermal ink jet printhead is frequently caused by the interaction of droplets jetted from the nozzles with ink accumulated on the surface of the array of nozzles. Since thermal ink jet inks generally are water-based compositions, the accumulation of ink on the printhead surface can be reduced or eliminated by treating the surface so as to render it hydrophobic and non-wetting by, for example, coating the printhead surface in the vicinity of the nozzles with a water repellent material. A suitable water repellent material generally should form films well, be easily applied to a surface, exhibit excellent adhesion to the materials from which ink jet printheads are typically made, form uniform, smooth films of thicknesses of a few microns or less, exhibit good abrasion resistance to be able to withstand frequent wiping during maintenance as well as normal wear from the jetting process, and be capable of being applied to the surface of the array of nozzles without contacting the walls of the nozzles.
Coatings for ink jet nozzles are known. For example, U.S. Pat. No. 4,643,948 (Diaz et al.), the disclosure of which is totally incorporated herein by reference, discloses a film coating for an ink jet nozzle plate comprising a partially fluorinated alkyl silane and a perfluorinated alkane.
In addition, U.S. Pat. No. 3,946,398 (Kyser et al.), the disclosure of which is totally incorporated herein by reference, discloses a recording apparatus which includes a writing fluid source feeding a drop projection means which ejects a series of droplets of writing fluid from a nozzle in a discontinuous stream with sufficient energy to traverse a substantially straight trajectory to a recording medium. The volume of each droplet is individually controlled by electrical pulses applied to the projection means from an electronic driver. A plurality of such projection means may be employed and connected to control means whereby to print or form predetermined graphical intelligence patterns on a record medium. To prevent wetting of the printing head by the ink, water based inks are used and the printing head surface is coated with Teflon.
Further, U.S. Pat. No. 4,751,532 (Fujimura et al.), the disclosure of which is totally incorporated herein by reference, discloses an ink jet printhead wherein thermal energy and an electroelastic field are applied to ink held between two plate members to cause the ink to be jetted out from an orifice defined by the plate members wherein there is provided, on the orifice-side end portions of each of the plate members adjacent the orifice, a first area readily wettable by the ink and a second area away from the orifice which is less wettable by the ink. Examples of materials suitable for coating portions of the plate members include silicone-type resins and fluorocarbon-type resins.
Additionally, U.S. Pat. No. 4,368,476 (Uehara et al.) discloses ink jet printheads which are treated with a compound represented as RSiX.sub.3, wherein R is a fluorine containing group and X is halogen, hydroxyl or a hydrolyzable group. The ink jet printhead may contain a number of differing materials, and accordingly, it is difficult to provide uniform coating.
U.S. Pat. No. 4,734,706 (le et al.) discloses a printhead for an ink jet printer having a protective membrane formed over the ink orifice. A viscoelastic and ink-immiscible fluid is used to form the membrane over the ink orifice. The membrane may comprise a silicone oil such as polydimethylsilicone polymers. The membrane lies in a plane perpendicular to the direction of emission of ink drops, and provides a barrier between the ink orifice and the external atmosphere, thus inhibiting evaporation of ink and the entry of contaminants. Wetting of the exterior surface of the ink jet head by the flow of ink through the ink orifice is also inhibited.
U.S. Pat. No. 4,728,392 (Miura et al.) discloses an ink jet printer of the electro-pneumatic type wherein an inner surface of a front nozzle plate and an end face of a rear nozzle member may be coated with a thin layer of an ink-repellent material. The ink-repellent material may be an ethylene tetrafluoride resin such as Teflon, or a fluoride containing polymer. Miura et al. also discloses blowing air through a nozzle while an ink-repellent material is applied thereto to prevent clogging of the nozzle. The nozzle-containing face of Miura et al. is made from one material.
U.S. Pat. No. 4,5623,906 (Chandrashekhar et al.) discloses a surface coating for ink jet nozzles. The coating includes a first layer of silicon nitride, an intermediate layer graded in composition, and a top-most layer of aluminum nitride.
Other disclosures of interest with respect to ink jet printers include U.S. Pat. No. 4,378,564 (Cross et al.), U.S. Pat. No. 4,511,598 (Creagh), and U.S. Pat. No. 4,725,862 (Matsuzaki), the disclosures of each of which are totally incorporated herein by reference.
U.S. Pat. No. 5,136,310, the disclosure of which is totally incorporated herein by reference, discloses an ink-repellent coating for a front face of a thermal ink jet printhead. Alkyl polysiloxanes, chlorosilanes and other ink-repellent materials are used to treat thermal ink jet nozzles in order to control their wetting characteristics to improve jet directionality and to prevent accumulation of debris on the array face. An intermediate layer of silica formed between the ink jet face and the ink-repellent layer may be provided so that the ink-repellent layer is isotropically hydrophobic. A method for applying the ink-repellent coating is provided in which an intermediate layer is formed by electron beam evaporation or sputtering, and an ink-repellent layer is applied by dipping in solution, wiping, spray coating, or the like, while blowing air through the channels of the ink jet.
While known compositions and processes are suitable for their intended purposes, a need remains for ink jet printing apparatuses wherein the front face of the printhead is coated with a water repellent material. In addition, a need remains for coatings for ink jet printing apparatuses that are easy to apply and form films well. Further, there is a need for coatings for ink jet printing apparatuses that exhibit excellent adhesion to materials from which ink jet printheads are typically made, such as silicon materials and polyimides. There is also a need for coatings for ink jet printing apparatuses that can form uniform, smooth films of a thickness of a few microns or less. A need also remains for coatings for ink jet printing apparatuses that can be applied to the surface of a printhead having an array of jetting nozzles without coating the ink conveying channels that terminate on the surface of the printhead. Additionally, there is a need for coatings for ink jet printing apparatuses that exhibit good abrasion resistance and are capable of withstanding frequent wiping during maintenance of ink jet printheads as well as withstanding normal wear from the jetting process. Further, a need exists for ink jet printheads that prevent the accumulation of ink and other material on the nozzle-containing face and thus maintain good ink jet directionality. In addition, there is a need for ink-repellent coating materials for an ink jet printhead which render the nozzle containing surface of the printhead uniformly ink-repellent even when the nozzle-containing surface is made from a plurality of different materials. There is also a need for coating materials for ink jet printheads which are compatible with glycol-containing inks, are stable over long periods of time and are free from unwanted material build-up during deposition on the nozzle face. A need also exists for coatings for ink jet printing apparatuses that retain ink repellent properties in the presence of ink additives such as surfactants that are included in the ink to promote rapid ink drying times. Further, there is a need for coatings for ink jet printing apparatuses that are of low cost and easy to apply and that result in a good yield of high quality, reproducible coatings.